1. Field of the Invention
The invention pertains to sensors formed in microstructures or nanostructures, and especially to the tests for validating the working of such sensors.
An increasing number of items used in daily life integrate chips provided with micro-electro-mechanical systems (MEMS) or nano-electro-mechanical systems (NEMS). Accelerometers or gyrometers in particular include inertial MEMS sensors.
With the improvement of silicon-etching methods and the search for greater circuit densities, chips integrating nano-electro-mechanical systems are undergoing many developments. A nanosystem may include especially a resonant sensitive sensor element. The sensitive element is often integrated into a CMOS architecture and is generally associated with an excitation electrode and a measuring electrode. The sensitive element is mobile with one degree of freedom for which it has a resonance frequency. The sensitive element has one of its dimensions typically ranging from 50 to 500 nm. This reduction of scale of the sensitive elements is supposed to increase their density of integration in wafers, for example silicon wafers, as well as their performance and thus reduces the unit cost of each chip. The integrity and the properties of the sensitive elements must however be tested throughout the manufacturing process. A simple visual control by an electronic scanning microscope proves to be insufficient to determine whether a sensitive element is functional. The tests thus aim at determining the resonance frequency and the quality factor of the sensitive element. The tests comprise firstly the excitation of the sensitive element and, secondly, measurements of motion of the sensitive element.
2. Description of the Prior Art
A known technique used to excite sensitive elements uses an electronic testing apparatus provided with special tips.
This technique is the only one used for sensors formed in a top-down approach on an entire silicon wafer, i.e. the wafer before the different chips are separated from one another. Each excitation electrode is provided with a contact against which a tip of the test apparatus comes into electrical contact. The tips apply AC voltage to the contacts to generate a movement of the sensitive parts according to their degree of freedom. However, each contact occupies a surface area that is approximately 20 to 100 times greater than the surface area of a sensitive element. A reduction in the dimensions of the sensitive elements therefore does not increase the integration density except in a small measure, and the limiting elements then become the contacts.
The contacts may be re-used to connect an associated electronic circuit. This means that the contacts are functional during the lifetime of the chip. However, in technologies where the electronic circuitry is made in the same silicon wafer as the sensitive elements (using what are called ‘in IC’ technologies), these contacts no longer have any functional utility during the lifetime of the chip. Furthermore, the formation of such contacts calls for numerous technological steps, giving rise to excess cost and an increase in the duration of the manufacturing process. The number of technological steps used to form the contacts may even be greater than the number needed to form the sensitive elements themselves. The contacts are usually shaped at the beginning of the chip-manufacturing process. The formation of the contacts however induces many random factors in the subsequent making of the sensitive elements, which is a more difficult operation, and this increases the discard rate. The presence of metal in the contacts is also a limiting factor for the remaining part of the manufacturing process. The contacts are themselves a non-negligible source of parasitic phenomena during the test: these contacts may induce parasitic capacitances with a magnitude that may be several hundreds of times greater than the value of the detection capacitance of a capacitive sensitive element.
In chip layouts on a same wafer (in IC technology), the sensitive elements may have no dedicated contacts. Connection tracks of the integrated circuit are then used to excite the sensitive elements. An architecture of this kind then cannot be used to test the sensitive elements independently of the integrated circuit. It is then impossible to separate causes of malfunction or to verify the effect of the integrated circuit on the functioning of the sensitive elements.
To form sensors using the bottom-up process, the problems encountered during the test phases are even more inconvenient. The sensitive elements made in the bottom-up approach are generally smaller in size and their performance characteristics are more difficult to measure. In experimental embodiments, such sensitive elements are made before the contacts are formed. In practice, it proves to be almost impossible to then form the contacts without destroying the sensitive elements, because of the brittleness of these sensitive elements. The contacts furthermore induce parasitic capacitances whose magnitude is even more of an inconvenience than it is for sensors made in the bottom-up approach.
The techniques of optical measurement of the excited sensitive elements prove to be unsuited for microstructures and even more so for nanostructures. The optical beams indeed have a minimum size of about five micrometers, which may correspond to several times the size of the sensitive element in motion.
A number of studies thus propose the use of a scanning electronic microscope to measure the excited sensitive element. Such microscopes emit an electron beam whose size may be reduced to 5 nm. The publication by Gilles Megherbi, Raynaud, Parrain, Mathias, Leroux and Bosseboeuf “Scanning electron microscopy for vacuum quality factor measurement of small-size MEMS resonators” 3 Dec. 2007, describes especially different methods of measurement implemented in a scanning electronic microscope. The excited sensor is subjected to an electron beam emitted by the microscope. The impact of this beam induces an emission of secondary electrons by the sensor. These secondary electrons are received by a detector and analyzed in such a way as to identify a shift of the excited sensor. Parameters of the sensors such as the amplitude of its shift or the resonance frequency of the sensitive element could be determined. However, a measuring method of this kind has drawbacks.
Furthermore, most testing methods cannot be implemented except at a very late stage in the manufacturing process, where the excitation of a sensitive element implies for example a preliminary connection of the functional electron circuits in implantations on a same wafer. Thus, most of the steps of the manufacturing method must be executed before any detection of a defect affecting the first steps. Furthermore, it is difficult to systematically determine the reliability of an excitation by electrical contact.